1. Field of the Invention
The present invention relates to a UPS (uninterrupted power supply) system having a function of parallel operation. Particularly, the present invention relates to the UPS system functioned to control a plurality of UPS units with identical or different capacities in parallel operation. More particularly, the present invention relates to the UPS system functioned to control each UPS unit depending upon its capacity ratio such that each UPS unit can provide an output current according to the capacity ratio.
2. Description of the Related Art
As the capacity of load is continuously expanded, there still exists a need for further expanding the capacity of the UPS system in satisfying such a demand for expanding the capacity of load. However, there are two conventional manners in expanding the total capacity of the UPS system: one is increasing the capacity of a single UPS unit; and the other is connecting several small-capacity UPS units in parallel. In general, the manner of connecting the small-capacity UPS units in parallel, however, is better than that of increasing the capacity of a single UPS unit, since the parallel connection of the small-capacity UPS units is expandable in adjusting the total capacity of the UPS system according to needs and has a high degree of reliability of the UPS system in operation.
However, several problems must be considered in designing the parallel connection of the small-capacity UPS units for using in a UPS system. First, with regard to the operational problem of such a UPS system, the load currents distributed among the individual UPS units must be equal to avoid occurring overload on some of the UPS units. Another problem is a circular current occurring among the UPS units which may result in an increase of power loss and which may lower the total operational efficiency of the UPS system. In addition, the circular current may further result in occurring overload in some of the individual UPS units.
Typically, the conventional control method for the UPS system having a function of parallel operation includes a master-slave control method, a concentrated distribution control method, a logical distribution control method, a ring control method, and a droop control method.
Firstly, the master-slave control method requires operating one master UPS unit in a voltage-control mode so as to regulate its output voltage and to lower its harmonic distortion. The other UPS units are formed as slave UPS units operated in a current-control mode so as to equally divide the load current. The master-slave control method must utilize a load-current distribution circuit which is adapted to equally distribute the load currents to the UPS units. Accordingly, there is a need of providing a communication line among the UPS units for transmitting the current distributional information.
The concentrated distribution control method is similar to the master-slave control method. However, the concentrated distribution control method requires operating all of the UPS units in current-control mode. However, the concentrated distribution control method must utilize a load-current distribution circuit which is used to determine the distributed load current of each UPS unit. Each of the UPS units must be controlled by an outer voltage control loop and an inner current control loop so as to regulate the output voltage and to lower the harmonic distortion.
The logical distribution control method requires detecting each output current of the individual UPS units and sending them to each control circuit of the individual UPS units. Thus, the control circuit can calculate a mean value of the total output currents of the individual UPS units acting as a reference signal and utilizes a current-control mode to control the output current of the individual UPS unit to be identical with the reference signal. Accordingly, the logical distribution control method accomplishes the distribution of the output currents of the individual UPS units.
The ring control method is similar to the logical distribution control method, but this method need not collect each output current of the UPS units. However, the ring control method only requires a first UPS unit to retrieve an output current of a final UPS unit and each middle UPS unit to retrieve an output current of a preceding UPS unit. Each retrieved output current is acted as a reference signal. All control circuits of the UPS units are arranged in a ring structure. The UPS unit utilizing the current-control mode can control its output current to be identical with the reference signal. Accordingly, all of the output currents of the UPS units are identical.
As is apparent from the above description, these four methods require providing a communication line among the UPS units for transmitting the current distributional information. However, the current distributional information during communication is susceptible to be interfered with, such that the reliability of the UPS system may be poor.
Another control method for the UPS system having a function of parallel operation is the droop control method which is implemented by a PQ (real and reactive power) control method. The PQ control method requires each UPS unit to control an output real power (P) and an output reactive power (Q) by controlling the phase and amplitude of its output voltage, respectively. In order to stabilize the UPS system in operation, the droop control method requires both of frequency and amplitude of the output voltage of the UPS unit to be in inverse proportion to its output current as if there is a negative slope of relationship therebetween. Thus, such a control method is so called the “droop control method.” In the droop control method, the output voltage of the UPS unit will decrease as its output current increases. Accordingly, the output currents of the UPS units are not identical. During increasing the negative slope, each of the UPS units has a better effect upon the equalized output current, but the voltage regulation of the UPS unit is degraded. Conversely, during decreasing the negative slope, the voltage regulation of the load voltage is improved, but each of the UPS units has a poor effect upon the equalized output current. Although it would be an advantage to omit transmitting the current distributional information among the UPS units in the droop control method, the output current equalization of the UPS units and voltage regulation of the load voltages are degraded.
It would be an advantage to arrange the UPS units with different capacities in parallel and to control each UPS unit according to its capacity, thereby combining the UPS units precisely and flexibly to match the load capacity, and reducing the installation cost of the UPS system. As can be seen in the previous description, the control method of the UPS system having a function of parallel operation is not suitable for the UPS units with various capacities but only suitable for the UPS units with identical capacity.
As is described in greater detail below, the present invention provides a UPS system having a function of parallel operation. The UPS system functions to control each UPS unit depending upon its capacity ratio, such that each UPS unit can be distributed to provide an output current according to the capacity ratio. Accordingly, the UPS system of the present invention can omit transmitting the current distributional information among the UPS units and can effectively increase a degree of reliability, simplify its control circuit, and improve its voltage regulation.